/Engine/Source/Runtime/Experimental/Chaos/Public/Chaos/Collision/CollisionKeys.h
This key only needs to be uinque within the context of a particle pair. There is no guarantee of global uniqueness. This key is only used by the FMultiShapePairCollisionDetector class which is used for colliding shape pairs where each shape is actually a hierarchy of shapes.
class FCollisionParticlePairConstraintKey